In the art of digital printing, such as with ink-jet printers or electrophotographic "laser" printers, the use of page description languages (PDL's) is well known. The PDL (or other format, such as a facsimile format) represents a system in which data representative of an image to be printed can be retained in a relatively small amount of memory. However, the printing hardware, such as an ink-jet printhead or modulating laser, requires "bitmap data," that is 1's and 0's, which are directly operative of the hardware to form the image. Most digital printing systems in current use include a software utility called a decomposer, also known as an interpreter, which performs the task of converting image data in a PDL or other format to the bitmap data directly useable by the hardware. Common types of PDL include PostScript.TM. and the various versions of PCL.
In any digital printing apparatus, the functions of the decomposer represent a "heavyweight" consumer of processor bandwidth. That is, whether the processor associated with the printing apparatus is a microprocessor or workstation such as a SPARC station, most of the activities of the processor in the printing process will be given over to the functions of the decomposer processing the PDL image data. Thus, in designing a digital printing system comprising hardware and software, it is generally desirable not to divert the processor from its essential task of operating the decomposer.
One software task which is often performed by the processor in a digital printing apparatus is "buffer clearing." In brief, once the decomposer, operated by the processor, outputs a quantity of bitmap data for use by the hardware, this bitmap data must be temporarily retained in a buffer until the precise time (such as when printing a particular page) that the hardware needs the data. Thus, large quantities of bitmap data must move in and out of the buffer in the course of printing a multi-page document, as the bitmap data for various pages are decomposed and then used to operate the hardware. However, once the page associated with the bitmap data has been printed and the bitmap data is no longer needed in the buffer, the space within the buffer that had been occupied by the bitmap data must be cleared, or erased, so that the space in memory can receive bitmap data for a subsequent image to be printed. Generally, if the space in the buffer were not cleared in this way, the bitmap of the subsequent image would simply be superimposed on the data of the previous image in the same space in the buffer, yielding nonsensical image data.
In general, the operation of buffer clearing simply involves the writing of a large series of zeroes (or other data consistent with a blank image) into the place in the buffer where a subsequent image is to be written. As mentioned above, this necessary step of "buffer clearing" can represent a consumer of processor bandwidth which competes with the activities of the processor in operating the decomposer. The present invention is directed to a system whereby this action of buffer clearing only minimally consumes processor bandwidth.